Zoology 10th Edition by Stephen Miller,John Harley

Exercise 1

The skin of a mammal is protective and is responsible for many mammalian characteristics. Epidermal derivatives of the skin include hair, claws, and glands (including mammary glands). Hair functions in the sense of touch, as insulation, and in communication. Unlike other amniotes, the teeth of mammals are specialized for different functions, and tooth structure is used by zoologists to characterize mammalian taxa. Mammals possess four-chambered hearts and separate pulmonary and systemic circuits. Circulation through the placenta provides the fetus with nutrients and promotes gas exchange. Mammals excrete primarily urea and have a long loop of the nephron to help conserve water. Mammals rely on behaviors, pheromones, sound, and the sense of touch for communication with other animals. Monotremes are oviparous. Other mammals are viviparous. Most female mammals undergo estrus, which is a time when females are receptive to males. Primate females undergo menstrual cycles, which involve the preparation of the uterus to receive the fertilized egg. Following fertilization, gestation periods may last from eight days (some marsupials) to 19 months (African elephant).
What modifications of the general circulatory pattern of adult mammals are found in fetal mammals? What do these modifications accomplish?